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If s is not NULL, the wctomb() function converts the wide
character wc to its multibyte representation and stores it at the
beginning of the character array pointed to by s. It updates the shift
state, which is stored in a static anonymous variable known only to the
wctomb() function, and returns the length of said multibyte
representation, that is, the number of bytes written at s.

The programmer must ensure that there is room for at least MB_CUR_MAX
bytes at s.

If s is NULL, the wctomb() function resets the shift state, known
only to this function, to the initial state, and returns nonzero if the
encoding has nontrivial shift state, or zero if the encoding is stateless.

If s is not NULL, the wctomb() function returns the number of
bytes that have been written to the byte array at s. If wc can
not be represented as a multibyte sequence (according to the current locale),
-1 is returned.

If s is NULL, the wctomb() function returns nonzero if the
encoding has nontrivial shift state, or zero if the encoding is stateless.

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